World War III is a term used to describe a hypothetical conflict on the scale of World War II or larger. Most usages of the term assume the use of weapons of mass destruction such as nuclear weapons or biological weapons.

 Historical close calls

Before the collapse of the Soviet Union and end of the Cold War, an apocalyptic war between the United States and the USSR was considered likely. The Cuban missile crisis in 1962 is generally thought to be the historical point at which the risk of World War III was closest. Other potential starts have included the following (see External links below for further examples):

• 1948-1949 - Berlin Blockade: The USSR blockaded Western Berlin in an attempt to remove America, France and Great Britain from Berlin. Some American politicians suggested an invasion of East Germany, however Truman was dissuaded from this by analysts saying that the risk and fallout of WWIII would be too great. (The Allies dealt with the Berlin Blockade with the Berlin Airlift, which was ultimately successful).
• July 26, 1956 - March, 1957 - Suez Crisis: The conflict pitted Egypt against an alliance between the French Fourth Republic, the United Kingdom and Israel. When the USSR threatened to intervene on behalf of Egypt, the Canadian Secretary of State for External Affairs Lester B. Pearson feared a larger war and persuaded the British and French to withdraw. The Eisenhower administration, fearing a wider war, had applied pressure to the United Kingdom to withdraw, including a threat to create a currency crisis by dumping US holdings of British debt.
• October 27, 1962 - Cuban Missile Crisis: The conflict pitted the United States against an alliance between the USSR and Cuba. The USSR was attempting to place several launch sites in Cuba in response to the United States installation of missiles in Turkey. The United States response included dispersal of Strategic Air Command bombers to civilian airfields around the United States and war games in which the United States Marine Corps landed against a dictator named "ORTSAC" (Castro spelt backwards). For a brief while, the U.S. military went to DEFCON 3, while SAC went to DEFCON 2. The crisis peaked on October 27, when a U-2 (piloted by Rudolph Anderson) was shot down over Cuba and another U-2 flight over Russia was almost intercepted when it strayed over Siberia, after Curtis LeMay (U.S. Air Force Chief of Staff) had neglected to enforce Presidential orders to suspend all overflights.
• October 24, 1973 - Yom Kippur War: As the Yom Kippur War was winding down, a Soviet threat to intervene on Egypt's behalf caused the United States to go to DEFCON 3.
• November 9, 1979 - False 'Soviet First Strike' Alarm: The US made emergency retaliation preparations after NORAD saw on-screen indications that a full-scale Soviet attack had been launched. No attempt was made to use the "red telephone" hotline to clarify the situation with the USSR and it was not until early-warning radar systems confirmed no such launch had taken place that NORAD realized that a computer system test had caused the display errors. A Senator inside the NORAD facility at the time described an atmosphere of absolute panic. A GAO investigation led to the construction of an off-site test facility, to prevent similar mistakes subsequently.
• September 26, 1983 - False 'US First Strike' Alarm: Soviet early warning systems showed that a US ICBM attack had been launched. Colonel Stanislav Petrov, in command of the monitoring facility, correctly put the warning down to computer error and did not notify his superiors.
• November 1983 - Exercise Able Archer: The USSR mistook a test of NATO's nuclear-release procedures as a fake cover for a NATO attack and subsequently raised its nuclear alert level. It was not until afterwards that the US realized how close it had come to nuclear war. At the time of the exercise the Soviet Politburo was without a healthy functioning head due to the failing health of then leader Yuri Andropov, which is thought to have been one of the contributing factors to the Soviet concern over the exercise.
• January 25, 1995 - Norwegian Rocket Incident: A Norwegian missile launch for scientific research was detected from Spitsbergen and thought to be an attack on Russia, launched from a submarine five minutes away from Moscow. Norway had notified the world that it would be making the launch, but the Russian Defense Ministry had neglected to notify those monitoring Russia's nuclear defense systems.

In addition to the above there are two other points during the Cold War that could have resulted in world war. These, however, are not generally listed as they do not relate to the United States-Soviet Union rivalry, but rather the events following the Sino-Soviet Split of 1960. The ideological split between Maoist communists (represented primarily by China) and Stalinist communists (represented primarily by the Soviet Union) divided the entire communist movement worldwide — which controlled governments or significant rebel factions on most continents. Thus a war between China and the Soviet Union may well have resulted in world war, while not necessarily involving the U.S. and the capitalist west. The two points the communist powers almost entered into all-out war over were:

• March 1969, when border clashes broke out between Soviet and Chinese troops over Zhen Bao Island in the Ussuri River (Sino-Soviet border conflict). In total, the Soviets suffered about 90 casualties to 800 for the Chinese (these numbers are based on Soviet claims). At the time there were almost one and a half million troops deployed along the border.
• 1978 and 1979, in which pro-Soviet Vietnam invaded pro-China Cambodia and removed Pol Pot. China in turn invaded Vietnam in retaliation and the Soviets denounced this action strongly, although it fell short of taking action. The next year the Soviets invaded Afghanistan and the Chinese claimed this was a continuation of a strategy of encircling China with Soviet allies that had begun the previous year with the Vietnamese invasion of Cambodia.

Finally, since the end of the Cold War, there have been at least two points in the decades-long conflict between India and Pakistan over Kashmir which almost escalated into nuclear conflict.